The groundwater budget: A tool for preliminary estimation of the hydraulic connection between neighboring aquifers

Accepted Manuscript Research papers The groundwater budget: a tool for preliminary estimation of the hydraulic connection between neighboring aquifers Stefano Viaroli, Lucia Mastrorillo, Francesca Lotti, Vittorio Paolucci, Roberto Mazza PII: DOI: Reference:

S0022-1694(17)30743-6 https://doi.org/10.1016/j.jhydrol.2017.10.066 HYDROL 22346

To appear in:

Journal of Hydrology

Received Date: Revised Date: Accepted Date:

3 October 2017 25 October 2017 26 October 2017

Please cite this article as: Viaroli, S., Mastrorillo, L., Lotti, F., Paolucci, V., Mazza, R., The groundwater budget: a tool for preliminary estimation of the hydraulic connection between neighboring aquifers, Journal of Hydrology (2017), doi: https://doi.org/10.1016/j.jhydrol.2017.10.066

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The groundwater budget: a tool for preliminary estimation of the hydraulic connection between neighboring aquifers Stefano Viarolia, Lucia Mastrorilloa, Francesca Lottib, Vittorio Paoluccic, Roberto Mazzaa a

Science Department, Roma Tre University, Largo S. Leonardo Murialdo 1, 00146 Rome, Italy.

[email protected] b

Kataclima S.r.l., Via Cassia 92, 01019 Vetralla, Italy. [email protected]

c

Ferrarelle S.p.A., Contrada Ferrarelle, 81053 Riardo, Italy. [email protected]

Corresponding author: Stefano Viaroli; Largo S. Leonardo Murialdo 1, 00146 Roma, Italy. email [email protected]; phone 0657338085; ORCID: 0000-0002-2521-7463

Abstract Groundwater management authorities usually use groundwater budget calculations to evaluate the sustainability of withdrawals for different purposes. The groundwater budget calculation does not always provide reliable information, and it must often be supported by further aquifer monitoring in the case of hydraulic connections between neighboring aquifers. The Riardo Plain aquifer is a strategic drinking resource for more than 100,000 people, water storage for 60 km2 of irrigated land, and the source of a mineral water bottling plant. Over a long period, the comparison between the direct recharge and the estimated natural outflow and withdrawals highlights a severe water deficit of approximately 40% of the total groundwater outflow. A groundwater budget deficit should be a clue to the aquifer depletion, but the results of long-term water level monitoring allowed the observation of the good condition of this aquifer. In fact, in the Riardo Plain, the calculated deficit is not comparable to the aquifer monitoring data acquired in the same period (1992-2014). The small oscillations of the groundwater level and the almost stable streambed spring discharge allows the presumption of an additional aquifer recharge source. The confined carbonate aquifer locally mixes with the above volcanic aquifer, providing an externally stable recharge that reduces the effects of the local rainfall variability. The combined approach of the groundwater budget results and long-term aquifer monitoring (spring discharge and/or hydraulic head oscillation) provides information about significant external groundwater exchanges, even if unidentified by field measurements, and supports the stakeholders in groundwater resource management.

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Keywords: Groundwater budget; Volcanic aquifer; Carbonate aquifer; effective infiltration; Groundwater exchanges; Long-term aquifer monitoring.

1 Introduction The increasing imbalance between the water supply and water demand in many parts of Europe, potentially exacerbated by changes in the climate during the past few decades (Di Matteo et al., 2010); water availability; and water scarcity have progressively emerged as key issues in national and EU water policy (Water Framework Directive; 2000/60/EC; European Commission, 2015) and strategy (European Commission, 2007; European Commission, 2012; European Commission, 2013). Many studies approach the problem of correct exploitation through quantification of the safe yield for an aquifer. Safe yield is commonly defined as the attainment and maintenance of a long-term balance between the amount of groundwater annually withdrawn and the annual amount of recharge (Han et al., 1997; Sophocleous, 2000). It follows that the available groundwater depends on how changes in recharge and discharge affect the surrounding environment (Alley and Leake, 2004). It is possible to derive balance equations to relate explicitly the sustainable pumping rate to the natural groundwater recharge (Zhou, 2009 and references therein), where the safe yield of an aquifer is often calculated as a percentage of the natural recharge. Most water management authorities pay higher attention to the results of the groundwater budget. In fact, building groundwater balances helps to combine and structure the key components of the natural hydrological cycle (without human pressures) and the relevant inputs and outputs due to human interventions (e.g., abstractions and returns) into a coherent framework. The different components of the groundwater balance convey information on the state of the inputs and outputs at reference times and their changes according to different reference periods. These components are dynamic, subject to trends and influenced by natural (variation in the amount and timing of precipitation and temperature) and anthropogenic drivers (e.g., water abstraction, flow regulation, and land-use change). Groundwater balances are differently applied today in the European catchment, depending mostly on the national laws. In Italy, water balances are developed according to the Guidelines for the Preparation of the th

Basin Water Balance, reported in the Ministry of Environment Protection Decree of 28 July 2004 and following an update in Legislative Decree 152/2006.

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Decreases in recharge or overexploitation could cause progressive depletion of the groundwater resource and a negative budget discrepancy (Calderhead et al., 2012; Marechal et al., 2006; Zhou, 2009). Over the long-term, the comparison between the amount of recharge and the total outflows (natural outflows and withdrawals) can provide information about the degree of groundwater resource exploitation (Capelli et al., 2005). Generally, the overexploitation (total outflows ≥ natural recharge) results in groundwater depletion associated with detrimental effects, such as well yield reduction, pumping cost increases, spring discharge decreases and groundwater-level decline (Custodio et al., 2016a, 2016b; Foster et al., 2004; Konikow, 2015). However, the groundwater budget could not be calculated without the complete knowledge of the boundary condition of a hydrogeological basin. If the considered system is bounded by clear groundwater divides (no flux boundaries), their positions define the basis of any water management decisions, which entail quantifying inflows and outflows (Genereux et al., 2005). The groundwater exchanges with external basins can occur, especially where the aquifer boundaries are not clearly defined, and thus, it becomes more difficult to properly assess the groundwater inflows and outflows (Filippini et al., 2015). In some cases, groundwater exchanges are negligible compared to the direct recharge; conversely, in other cases, they could be consistent and generate effects on water quality or significant discrepancies in the groundwater budget (Mastrorillo and Petitta, 2014). Moreover, in some thick and fractured aquifers, contributions of vertical flow from depth could be significant in terms of available resources. In these cases, the exploitation could have low or negligible impacts and groundwater levels and spring flow rates might remain unchanged due to the hydraulic connection with neighboring aquifers. These hidden and scarcely quantifiable inflows could constitute a major portion of the available groundwater resource (Carrillo Rivera, 2000). In conclusion, the balance equation depends on three main components: the natural recharge (effective infiltration), the anthropogenic induced activities and the relationship of the studied system with neighboring basins (external groundwater inflow and outflow). In the presence of significant external groundwater exchanges, not assumed in the conceptual model, the results of only the groundwater budget calculation could not be reliable precisely because of the high uncertainty of the non-measurable groundwater exchanges. The combined results of the groundwater budget and the aquifer monitoring (spring discharge or hydraulic head measurements) over a long time could provide more information about the aquifer conditions and could support the definition of a new hydrogeological conceptual model.

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These circumstances seem to be present in the Riardo Plain aquifer, which is a strategic drinking resource for more than 100,000 people, water storage for 60 km2 of irrigated land, and the source of a mineral water bottling plant (Viaroli et al., 2016c). In this area, located in southern Italy, water supply operators have expressed concerns about the potential impact of the strong and prolonged exploitation for conjunctive purposes on the future availability of groundwater resources (Autorità di Bacino dei Fiumi Liri – Garigliano e Volturno, 2008). The present study aims to contribute to improving the knowledge of the recharge processes of the Riardo Plain aquifer in order to verify the actual aquifer conditions and continue its exploitation without risking abrupt groundwater depletion. For this purpose, a simplified approach was used to evaluate the average annual and monthly groundwater budgets over twenty-three years (1992–2014) and to calculate the monthly mean distribution of the effective infiltration. The yearly mean groundwater withdrawals were also evaluated. The obtained results were compared with both the groundwater levels/outflow variations and the precipitation variability, measured in the same monitoring interval. A new simplified hydrogeological conceptual model of the Riardo Plain aquifer was also proposed to better define the hypothetical groundwater recharge processes. Given the complexity of the hydrogeological setting of the study area, the groundwater recharge model suggested was presented in its most simplified form and will inevitably have to be deepened by further studies. Nevertheless, the first results achieved are already enough for supporting stakeholder decisions and directing future investigations. The proposed case study highlights the groundwater budget calculation of the aquifer affected by the hydraulic connection between neighboring aquifers, such as the Riardo Plain aquifer, and is useful for the preliminary identification of the groundwater exchanges, which are often very complicated to quantify. In fact, the underestimation or the overestimation of the external exchanges could significantly affect groundwater withdrawal management, thus increasing the socioeconomic costs and the risk of overexploitation. According to the results of this study, a combined approach with the groundwater budget calculation and long-term aquifer monitoring (spring discharge and/or groundwater level) is recommended in order to reduce this uncertainty and to better evaluate the sustainability of the withdrawals for wise water management.

In this approach, the aquifer recharge and exploitation are quantified by the groundwater budget calculation, whereas the aquifer conservation status is defined by the long-term monitoring. This coupled

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control allows the identification of potential anomalies related to not otherwise recognizable groundwater inflows or outflows.

2. Geological and hydrogeological framework The concerned area is located in the northern Campania Region (southern Italy) and corresponds to the eastern slope of the Roccamonfina Volcano and the Riardo Plain, an intermountain basin formed during the Apennine chain orogenesis. The carbonate southern Apennines are mainly made up of the Lias–Cretaceous shelf limestone succession with a thickness of approximately 1500 m (D’Argenio and Pescatore, 1962). From the Messinian to early lower Pliocene, the sedimentary succession was affected by a compressive tectonic stage (orogenesis of the Apennine chain), especially during the Upper Tortonian, when the study area corresponded to the foredeep basin, and sin-orogenic flysch deposition occurred (Selli, 1957). The widespread Miocene flysch units cover the older Mesozoic limestone successions and are the stratigraphic basement of the Roccamonfina Volcano (Capuano et al., 1992; Saroli et al., 2017). Subsequently, an extensional tectonic stage (Lower Pliocene-Upper Pleistocene) produced a horst and graben structure, which extended in the NW-SE direction and is dislocated in the NE-SW direction (Chiappini et al., 1998). The dislocation of the sedimentary basement formed the tectonic basin of the Riardo Plain, subsequently filled by volcanic units, Pleistocene–Holocene volcanoclastic deposits and actual alluvial units. The Roccamonfina Volcano activity started approximately 550 ka, triggered by the extensional tectonic activity, and ended approximately 150 ka. Surrounding carbonate and marls horsts of the Cretaceous age form the lateral margins of the basin in which the Roccamonfina has developed (Rouchon et al., 2008). The volcanic activity is subdivided into three main epochs (De Rita et al., 1997). The first epoch is mainly characterized by effusive activity with the emission of sub-saturated lava flows and by the growing of the stratovolcano and several small volcanic centers. The second activity epoch, started approximately 350 ka (Rouchon et al., 2008), is characterized by a succession of highly explosive eruptions and the emplacement of pyroclastic units (Giannetti and De Casa, 2000; Luhr and Giannetti, 1987). During the third phase, minor phreatomagmatic activity and latitic lava domes were emplaced (De Rita and Giordano, 1996; Giannetti, 1964, 1996). Borehole data in the Riardo Plain highlight the local absence of the flysch deposits in the eastern peripheral sector of the Roccamonfina Volcano and the direct deposition of the volcanic units over the carbonate

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basement. The thickness of the volcanic deposits in the Riardo Plain is extremely variable according to the pre-volcanic morphology and ranges from a few meters near the carbonate ridges to more than 250 m (detected in a borehole in the Ferrarelle S.p.A. bottling plant) moving toward the volcano edifice. Geophysical data (Capuano et al., 1992) suggested an increase in the deepening of the sedimentary basement in correspondence with the Roccamonfina Volcano edifice, where several hundred meters of lava deposits emplaced during the first phase of the volcanic activity are expected. The hydrostructure of the Roccamonfina Volcano hosts a regional centrifugal aquifer, which constitutes an open hydrogeological unit towards the surrounding plains. The Roccamonfina Volcano (S7 in Fig. 1) 2

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recharge area (approximately 350 km ) feeds several springs, up to a total discharge of 2.1 m /s (including the Savone linear spring of approximately 1.2 m3/s), and groundwater leakage is estimated to be 3

approximately 2 m /s (Boni et al., 1986). The volcanic hydrostructure is surrounded by five carbonate hydrogeological units (U1, U2, U3, S6 and the southern portion of the G2 units). Each carbonate hydrostructure mainly discharges towards productive springs located at its edge, with rates higher than 1 m3/s (Boni et al., 1986; Fig. 1). Recent hydrogeological studies on the Roccamonfina Volcano (Viaroli et al., 2016b) also suggest the distinction between the groundwater circulation in the caldera and the basal volcanic aquifer. In addition, along the eastern slope of the Roccamonfina Volcano, some low productive aquifers perched on a shallow aquitard (low permeability pyroclastic deposits) were identified (Viaroli et al., 2016a). These aquifers usually feed a few small, seasonal springs. The volcanic aquifer is recharged from the upper portion of the volcano edifice, and groundwater moves radially toward the Riardo Plain, where it feeds the Savone linear spring at an elevation between 90 and 40 m a.s.l. (Viaroli et al., 2016c). In the Riardo Plain, the volcanic aquifer overlaps a deeper fractured-carbonate aquifer, characterized by high secondary porosity. The carbonate aquifer is mainly recharged by the eastern portion of the volcano edifice (Capelli et al., 1999; Cuoco et al., 2010). The two aquifers can be clearly distinguished in the eastern sector of the plain, where the Flysch unit covers the carbonate substratum and plays the role of an aquiclude. Consequently, carbonate and volcanic aquifers show different hydraulic heads and opposite flow directions (Viaroli et al., 2016c). In contrast, hydraulic connections were recognized near the town of Riardo, where the intersection of different fault systems and the absence of the Flysch units allow the uprising of deep fluids (Cuoco et al., 2010). In this sector, the carbonate and volcanic hydraulic head have the same elevation

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(between 140 and 60 m a.s.l.), and the mixing with CO2 supersaturated fluids forms the local mineral water type. The geochemical composition of the volcanic aquifer confirms that the mineralization process depends only on the water–volcanic rock interaction. The deeper carbonate aquifer shows both carbonate and volcanic features (i.e., alkali metals, silica, Fe and As) as a consequence of the mineralization processes occurring during groundwater evolution. During the groundwater transfer from the recharge area (Roccamonfina Volcano) to the Riardo Plain, the groundwater interacts not only with volcanic units but also with carbonate rocks (Cuoco et al. 2010) wherever the elevation of the sedimentary basement reached the volcanic groundwater flow path elevation (Fig. 2). Under these conditions, the two aquifers (carbonate and volcanic) could then be considered a single, large basal groundwater circulation with chemistry variations according to the depth, called the “basal aquifer”. From the climatic point of view, the study area is characterized by a typical Mediterranean climate with dry and warm summers (from June to August) and a wet period that occurs during autumn, winter and spring (from September to April) (Ducci and Tranfaglia, 2008; Fiorillo, 2009). The mean monthly rainfall reaches a maximum during November (approximately 160 mm) and a minimum in July and August (approximately 30 mm). The mean annual rainfall measured in the northern Campania Region is approximately 1000 mm, with a coefficient of variation of approximately 0.3. The air temperature has a similar trend in all meteorological stations with some differences related to the ground elevation. The minimum temperatures were measured in January and February (approximately 7 °C) with a progressive increasing up to July and August (approximately 23.5 °C) ([dataset] Ferrarelle S.p.A., 2015; [dataset] Regione Campania. 2015a, 2015b).

2.1 Study area The study area corresponds to the eastern slope of the Roccamonfina Volcano and the western sector of the Riardo Plain, where hydraulic connections were recognized between the volcanic and the carbonate portion of the basal aquifer. The groundwater balance reference area has an extension of approximately 100 km

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(Fig. 3), and most of the boundaries were defined as the groundwater divides of the volcanic aquifer. The southeastern boundary corresponds to the outcrop of the M. Coricuzzo carbonate relief, which does not seem to be in connection with the aquifer in the plain (Viaroli et al., 2016c). In this area, the water table of the basal aquifer shows depths ranging from at least 200 meters on the volcano edifice to 0 m, where the Savone streambed spring outflow. In the western sector of the Riardo Plain, seven hydrogeological

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complexes were defined (modified from Allocca et al., 2007; Capelli et al., 1999; Mazza et al., 2013; Viaroli et al., 2016a), of which only five outcrop in the reference area. •

Alluvial complex: This complex is composed of reworked sediments deposited at the base of slopes or in the fluvial valley. These deposits could host aquifers due to the high primary porosity.

•

Dolomite complex (no outcropping in the reference area): This complex corresponds to the dolomite unit at the bottom of the sedimentary sequence. The dolomite complex is characterized by average secondary porosity; therefore, it plays the role of aquitard for the carbonate aquifer.

•

Limestone complex: This complex corresponds to the limestone units and is characterized by high secondary porosity according to fractioning and karst processes. This complex hosts the carbonate aquifer.

•

Flysch complex (no outcropping in the reference area): This complex is composed of sands and clays deposited in the foredeep basin. This complex is characterized by overall low hydraulic conductivity, and it plays the role of aquiclude between the volcanic and carbonate aquifers.

•

Lava complex: This complex corresponds to all lava and lava dome units, mainly characterized by high hydraulic conductivity due to cooling fractures. These units could also serve as a local aquiclude where they appear nearly unfractured.

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Fine grained pyroclastic complex: This complex is composed of lithified pyroclastic deposits and ashy matrix pyroclastic deposits. Due to its low hydraulic conductivity, this complex is a local aquitard separating aquifer layers hosted in the permeable volcanic complex

•

Coarse grained pyroclastic complex: This complex includes all other pyroclastic deposits, characterized by little higher porosity than the other pyroclastic units. This complex hosts the volcanic aquifer due to average primary porosity.

The groundwater budget parameters were calculated for the basal aquifer and recharged by direct rainfall infiltration of the Roccamonfina Volcano slope according to the Riardo Plain hydrogeological framework described above. This aquifer has proven to be rather productive, with transmissivity values ranging from -3

-2

10E (volcanic portion of the basal aquifer) to 10E (carbonate circulation) (unpublished aquifer pumping tests by Ferrarelle S.p.A.), and it is exploited intensively for agricultural and drinking water uses. Most of the Riardo Plain (approximately 60% of the study area) is used for agricultural purposes, especially for irrigated plantations such as fruit trees (more than 40% of the total area) and cereal crops (approximately 8%).

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The remaining area is covered by deciduous trees (more than 30%) in the upper section of the Roccamonfina Volcano or by scattered small urbanized areas (approximately 4%) (European Environment Agency, 2000). The most substantial well fields are located near Teano Town (Teano–S. Giulianeta, with a constant pumping rate of 240 L/s for drinking purposes) and the Ferrarelle S.p.A. mineral water bottling plant (mean withdrawal approximately 50 L/s) ([dataset] Ferrarelle S.p.A., 2016) (Fig. 3).

3. Materials and methods 3.1 Groundwater budget terms To calculate the groundwater budget of the Riardo Plain hydrogeological system, the prior spatial reference system and the temporal interval were both fixed. The boundaries of the Riardo Plain hydrogeological system were based on the bibliographic data described in the previous paragraphs. The recharge area, limited by the groundwater divides identified on the 2

potentiometric map by Viaroli et al. (2016c), covers approximately 100 km . The time interval for which the groundwater budget was calculated corresponds to 23 years (from 1992 to 2014). Given the large size of the area and the length of the observation period, inevitably, the inflow and outflow terms of the budget lack spatial and temporal homogeneity. The longest available time series were preferred, even if not always continuous, in order to omit the quantification of the aquifer storage variations, which can be neglected if compared to the other budget terms over such a long period (Custodio and Llamas, 2005). The next paragraphs of this chapter explain how the problems about heterogeneous data were overcome on a case by case basis. The groundwater budget terms were defined both at the mean monthly and at mean yearly scales according to the available data and distinguished based on the inflow and outflow terms. The unique inflow term corresponds to the effective infiltration calculated from thermo-pluviometric data. It was verified that lateral inflows in the aquifers from rivers are negligible. In fact, according to previous hydrogeological survey results (Università degli Studi Roma Tre, 1999; Università degli Studi Roma Tre, 2014; Viaroli et al., 2016b), the Savone River (unique perennial river in the reference area) is not in hydraulic connection with the aquifer along the volcano slope, where the low permeability, fine-grained pyroclastic complex outcrops (Fig. 3). In contrast, in the Riardo Plain, several streambed springs are present in correspondence with the outcrops of the aquifer complex (coarse-grained pyroclastic complex). No losing portions of the Savone River were detected in the field.

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Since most of the water pumped in the basin is used for irrigation, part of it can return to the aquifer by direct infiltration. In this work, the agricultural return flow was considered not relevant according to land use information. Approximately 70% of the irrigated areas corresponds to fruit trees, for which no return flow was assumed since these crops use drip irrigation techniques that eliminate irrigation losses (Marechal et al., 2006). The absence of paddy fields and irrigation channels allows this possible additional inflow term to be neglected. The outflow terms consist of natural outflows (discharge of Savone streambed springs) and artificial outflows (groundwater withdrawals). The main difficulties were found in the assessment of these terms because of both the lack of continuous monitoring of the Savone River baseflow and the absence of an official database about withdrawals for agricultural purposes. This set of problems is also discussed in the next paragraphs on a case by case basis. In conclusion, the classical groundwater budget equation (INPUT = OUTPUT ± ∆S) in this work is stated as follows: Ei = Nd + Wt ± ∆S where Ei (effective infiltration) is the parameter quantifying the amount of rainfall that is involved in the aquifer recharge; Nd (natural discharge) corresponds to the total discharge of the springs fed by the aquifer; Wt (artificial discharge) corresponds to the sum of the agricultural, industrial and drinking withdrawals; and ∆S is the aquifer storage variation, neglected in this work according to the long time interval of the budget calculation. 3.1.1 Inflow The groundwater budget inflow terms were computed from daily thermo-pluviometric data of local weather stations placed in the northern Campania Region. The data, acquired from several databases ([dataset] Ferrarelle S.p.A., 2015; [dataset] Regione Campania, 2015a, 2015b), have a time range of 15 years, from 2000 to 2014, except for the Riardo – Ferrarelle and Roccamonfina weather stations whose locations are shown in Fig. 3, which have a dataset recorded in the 1992-2014 interval. The monthly thermo-pluviometric dataset, resulting from the aggregation of the daily data at a monthly scale, presents gaps in each weather station due to different periods of activity or to technical malfunctions of the instruments. The missing data were reconstructed according to the correlation of the contemporaneous data

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of nearby stations. Only the correlations with a R > 0.85 were taken into account to fill the gaps. Finally, cumulate monthly rainfall and mean monthly temperature were calculated for all weather stations. The mean monthly actual evapotranspiration, the water surplus and the water deficit were calculated using Thornthwaite’s method (Thornthwaite and Mather, 1957) for each weather station. The punctual results of the water surplus were then geostatistically interpolated. Considering the existing correlation of the weather variables with the ground elevation, cokriging was applied using GIS to spatialize the data. The monthly effective infiltration (as the portion of the water surplus that can infiltrate) was calculated multiplying the monthly distributed water surplus with the coefficients of potential infiltration weighted with the slope (C.I.P.S.). The infiltration potential coefficients (C.I.P.) (Celico, 1988) evaluate the range of the amount (in percent) of the water surplus, which could infiltrate according to the physical properties of the outcropping soils or rocks (e.g., porosity and permeability). C.I.P. values were associated with the outcropping volcanic and sedimentary complexes described (Table 1) according to previous studies (Autorità di Bacino dei Fiumi Liri – Garigliano e Volturno, 2008; Celico, 1988). To include the effect of the morphology on infiltration, the raster map of C.I.P. distribution was multiplied by the slope map (S, in %) and calculated from the digital elevation model map with 50 meters of resolution in GIS. The raster maps were then combined via map algebraic operations; the C.I.P.S. were calculated cell by cell according to the following relationship:
. . . . =
. . .× 1 −

 100

The maps of the monthly distribution of the effective infiltration were realized, taking in account the distribution of the C.I.P.S. and the distributed monthly water surplus. The elaborations were clipped according the boundaries of the recharge area. 3.1.2 Outflows In the groundwater budget calculation, the recharge was compared with the main groundwater outflows: the natural discharge of Savone linear springs and the groundwater withdrawals for different purposes. It is assumed that the baseflow rate of the Savone River, measured at the closing section during dry periods, corresponds with the total groundwater natural outflows. Data regarding river discharge were collected during specific hydrogeological surveys (Autorità di Bacino dei Fiumi Liri – Garigliano e Volturno, 2008; Università degli Studi Roma Tre, 1999; Università degli Studi Roma Tre, 2014) carried out at least one week

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after the last significant rainfall event to exclude runoff from the assessment of the baseflow. From available data, it was possible to assess the groundwater natural outflow for each hydrogeological survey. Withdrawal data were collected from several databases and separately elaborated according to the different purposes. Data about drinking water withdrawals were collected from Autorità di Bacino dei Fiumi Liri– Garigliano e Volturno (2008), where information supplied from the technical bureau of the Municipalities and from the Aqueduct District database (ATO 2 Napoli – Volturno Regione Campania, 2003) is reported. Withdrawals for industrial purposes were calculated from the information of the “Industries and services census” ([dataset] ISTAT, 2011) at the municipal scale. The daily groundwater requirement for each employee was assessed according to the ATECO classification ([dataset] ISTAT, 2007) of all productive activities placed in the study area. The withdrawals of the mineral water bottling activity were included in this category. Information about withdrawals for agricultural purposes are not present in any specific database. The previous hydrogeological studies (Autorità di Bacino dei Fiumi Liri – Garigliano e Volturno, 2008; Capelli et al., 1999) assumed that the agricultural withdrawals were at least the difference between the water requirement for the plantations and the available water. The authors calculated the plantation water requirements on the different plant species distributed according to the land use information (European Environment Agency, 2000; [dataset] ISTAT, 1994; Nino and Fais, 2001). In the present study, groundwater withdrawal for irrigation purposes was set equal to the mean monthly water deficit calculated using Thornthwaithe’s method in agricultural areas and equal to zero elsewhere. The agricultural withdrawals were thus calculated from water requirements of the plantation without distinguishing among the different species. A small agricultural portion of the study area is irrigated through the Sannio Alifano Consortium, which takes water from outside the studied area, as since 2011. This external water contribution is taken into account in the budget calculation according to the information supplied by the consortium. 3.2 Aquifer monitoring The hydrogeological budget results were compared to the groundwater levels trend measured almost every 15 days since the ‘90s ([dataset] Ferrarelle S.p.A., 2016) in order to evaluate any possible effect of groundwater exploitation during the considered time interval. Among the several monitored points, two significant wells (PzC and PzV) placed in the Ferrarelle mineral water bottling plant (Fig. 3) were chosen. The hydraulic head level measured in the selected wells are characterized by the same oscillations in

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magnitude and in time measured in the other wells. In addition, these two wells provide the longest and most complete time series. PzV is an approximately 60 m-deep monitoring well tapping the volcanic portion of the basal aquifer. PzC is an approximately 250 m-deep monitoring well tapping the carbonate portion of the basal aquifer. 3.3 Climatic trend analysis The Standard Precipitation Index (SPI) (McKee et al., 1993; 1995) is a powerful, flexible index to analyze the occurrence and the magnitude of drought periods (World Meteorological Organization, 2012). The SPI was designed to quantify the precipitation deficit for multiple timescales according to the different water resource dynamics. The SPI is defined for each timescale as follows:

 =

 − ̅ 

where xi is the monthly rainfall amount, ̅ is the arithmetic mean of rainfall and  is the standard deviation calculated from the whole monthly time series. Positive SPI values indicate a greater than median precipitation (wetter periods), and negative values indicate less than median precipitation (drier periods) (McKee et al., 1993). A drought event occurs any time the SPI is continuously negative and reaches an intensity of at least -1. The event ends when the SPI becomes positive. The SPI can be calculated from 1 month up to 72 months, but statistically, 1-24 months is the best practical range of application (Guttman, 1998; 1999). Few studies have explored the correlation between the groundwater level variations (Mendicino et al., 2008; Bloomfield and Marchant, 2013; Bloomfield et al., 2015; Kumar et al., 2016; Lorenzo-Lacruz et al., 2017) or the spring discharge variations (Fiorillo and Guadagno 2010, 2012) to SPI. The authors suggested different statistical elaborations to standardize the aquifer oscillations, such as the Groundwater Resource Index (Mendicino et al. 2008), Standardized Groundwater Level Index (Bloomfield and Marchant, 2013) and Standardized Streamflow Index (Vicente-Serrano et al., 2012). The results of all methods, applied at the catchment scale over multiple long-term monitoring datasets, agree with the importance of having a high density of hydrological and climatic data to understand the effects of the precipitation variability according to the sub basin hydrogeological properties. In this study, the non-uniform spatial distribution and the scarcity of long-term hydrological monitoring datasets do not allow detailed statistical elaborations over the entire catchment. Therefore, the SPI was used

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to highlight the precipitation variability over the groundwater budget calculation period, also focusing on the presence of drought periods to compare with the aquifer oscillations. The climatic elaboration was realized by taking into account the precipitation time series of the Riardo– Ferrarelle meteorological station, which guarantees rainfall data from 1980 without significant interruptions ([dataset] Ferrarelle S.p.A., 2015). The available dataset was transformed into a normalized distribution and then standardized (i.e., mean SPI equal to 0). The rainfall data were elaborated using a 12 month time scale (SPI12) since it better represents long-term precipitation patterns and is better tied to groundwater resource dynamics (Fiorillo and Guadagno, 2010; World Meteorological Organization, 2012).

4. Results 4.1. Groundwater budget calculation The annual rainfall data, measured in the eight weather stations in the northern Campania Region, show a wide range (from approximately 580 mm in 2000 to approximately 1600 mm measured in 2009). The mean monthly rainfall data, calculated over the 2000/14 period, shows a similar trend for the eight stations (Fig. 4), with a decrease in monthly rainfall from January to August, a quick increase during the autumn, and maximum precipitation in November. The mean temperature trend is common to all monitored weather stations, characterized by cold winters and warm summers. Relative differences in the temperature are only related to the station elevation according to a linear relationship (R2 > 0.85). In Fig. 4, the monthly mean water surplus (Ws) is also represented, calculated according to Ws = P – Ev, where P is the mean monthly rainfall and Ev corresponds to the monthly actual evapotranspiration calculated using Thornthwaite’s method. The groundwater recharge is possible only when Ws is not zero. In all weather stations, the recharge period was identified between November and April (with one exception, starting in December). From May to October, the actual evapotranspiration exceeds the rainfall, with a consequent progressive depletion of the initial amount of soil water. During July and August, the available soil water turns to zero, and the higher evapotranspiration demand causes a water deficit. The water deficit and the available water in the soil are restored by the autumnal precipitation. At the Grazzanise weather station, the Ws was identified only from December to April due to a lower amount of autumnal rainfall. The geostatistical interpolation of the monthly water surplus and water deficit values calculated for each weather station allowed the extension of the values to the whole recharge area. The results of the cokriging

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between the eight weather stations in the 2000/14 period were compared to the results of the same procedure applied only to the Roccamonfina and Riardo–Ferrarelle meteorological stations, which began taking measurements in 1992. The comparison highlighted only minor differences and the usual overestimation of the water surplus (approximately 3%) in the elaboration using only two stations. Concerning the reliability of the elaboration, the monthly Ws and water deficit were calculated using the Roccamonfina and Riardo–Ferrarelle datasets over a twenty-three year (1992/2014) interval. The results are reported in Table 2. The spatial distribution of the C.I.P.S. (Fig. 5) allowed the identification of the areas where the effective infiltration prevails over the surface runoff. These areas correspond to the eastern portion of the study area where the alluvial deposits of the Riardo Plain outcrop and the zones where lava and limestone outcrop over the Roccamonfina Volcano flank. The C.I.P.S. values range from 93 to 21, with a mean value of approximately 55. The spatial distribution of the mean monthly effective infiltration (Fig. 6) in the study area was calculated, relating the C.I.P.S. raster with the raster distribution of the monthly mean water surplus. The summary of the mean monthly values of the recharge parameters is reported in Table 3. 2

Over the recharge area, approximately 100 km wide, the mean annual effective infiltration (233 mm/y) guarantees a renewable groundwater resource of 740 L/s per year. The mean annual effective infiltration calculated over the study area corresponds to approximately 25% of the mean annual precipitation, similar to the results of regional hydrogeological studies over the other perithyrrenic quaternary volcanic domains (Baiocchi et al., 2008; Boni et al. 1986, Capelli et al., 2005). The groundwater outflows from the hydrogeological system could be divided into natural discharges of the aquifer and anthropogenic withdrawals. The natural outflow is assumed to be represented by the Savone linear spring discharge at elevations between 120 and 40 m a.s.l. (Fig. 3). The baseflow of the Savone is approximately 470 L/s, with minimum discharges measured in July (315 L/s in 1998, 200 L/s in 2002 and 330 L/s in 2013). The withdrawals can be distinguished and calculated according to the different uses. The quantification of the agricultural withdrawals is one of the most important and complicated terms of the budget to be quantified (Tsanis and Apostolaki, 2009; Venezian Scarascia et al., 2006). Additional difficulties in agricultural withdrawal evaluation is the absence of an irrigation consortium which could provide local and reliable information on the water distribution on the entire study area and the absence of surface water resources, including artificial accumulation reservoirs, which correspond to the

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greatest source of water for irrigation (Todorovic et al., 2007). Nevertheless, it is important to underline that the knowledge about groundwater resources is far from accurate due to frequent non-authorized water abstraction for irrigation, especially in the southern regions (Todorovic et al., 2007). This study attempted to compare the monthly water surplus and the water deficit in the 1992/2014 period (Fig. 7). The groundwater extraction, required for the plantations life, was preliminarily and conservatively evaluated equal to the water deficit. The presence of a water deficit was detected in July and August, months in which evapotranspiration exceeds the rainfall amount but with no more water apparent in the soil. Thus, it seems reasonable to assume that irrigation is necessarily performed in order to avoid plant desiccation. The agricultural withdrawals were calculated according to the monthly water deficit distributed over the 60 km

2

irrigated areas (approximately 60% of the study area) (Fig. 3). The estimated groundwater withdrawals for agricultural purposes are summarized in Table 4. This outflow represents only a rough estimate since it is based on the local climatic conditions and not on the actual pumping rates of private wells. These values fluctuate from 655 L/s in 2003 (the driest year of the considered period) to 219 L/s in 1995, with an average value of approximately 420 L/s. Approximately 40 L/s shall be subtracted in the 2011/14 interval to take into account the irrigation water from the external area provided by the “Bonifica Sannio Alifano Consortium”. In conclusion, a net agricultural withdrawal of approximately 395 L/s was assumed. A limited number of industrial activities are located in the study area; the calculated industrial withdrawals is approximately 60 L/s. The mean value of the mineral water bottling activity was included in this withdrawal category, reaching a mean withdrawal of approximately 50 L/s during the study period. Most of the drinking wells that exploit the study aquifer fed the local population (approximately 30,000 people) with a rate of few liters per second (Table 5). Six highly productive wells with a total constant pumping rate of 240 L/s, located near the town of Teano (Teano – S. Giulianeta), are connected to the northern Campanian Aqueduct, which feeds three cities external to the studied area. The Teano–S. Giulianeta well field has been active since 1992, with a constant withdrawal rate during the study period. The potential number of people fed by the Riardo Plain hydrogeological system reaches 100,000 units, corresponding to a calculated withdrawal for human consumption of approximately 300 L/s. In Table 6, all parameters of the groundwater budget calculation are summarized. The estimation of the budget terms highlights a groundwater deficit of approximately 485 L/s. The calculated deficit is very high and surely wider than the calculation uncertainties.

4.2 Analysis of the precipitation trend 16

The computed SPI values are reported in Fig. 8. The drought and wet periods are defined according to the classification system proposed by McKee et al. (1993). The first portion (1980/85) of the precipitation is characterized by an alternate moderately wet (1 < SPI12 > 1.49) and moderately dry periods (-1.49 < SPI12 > -1). During the following 13 years, significant wet periods are not detected; only a moderately dry period between 1988/93 is evident. The time interval period of the groundwater budget calculation is a characterized variable pattern: an initial extremely dry period (SPI12 < -2) following a near normal/moderately dry period (0.99 < SPI12 > -0.99). Starting from 2008, there is a significant change in the precipitation regime, with the presence of extremely wet (SPI12 > 2) and very wet (1.5 < SPI12 > 1.99) periods. It is important to emphasize that significant wet periods are missing from 1985 to 2008 (23 years). The precipitation trend calculated in the Riardo Plain is similar to the results of other climatic studies realized in the same region. The extremely dry period detected in the first ’00 is the strictest drought period in magnitude and duration starting from 1930 (Fiorillo and Guadagno, 2010).

5. Discussion During the study period (1992-2014), the hydrogeological system in question provided a mean discharge of approximately 1200 L/s, calculated as the sum of natural and artificial outflows. In the same period, a mean effective infiltration of approximately 740 L/s was calculated. The results highlight a significant groundwater deficit (485 L/s), which is around 40% of the total discharge. In an intensively exploited hydrogeological system for conjunctive purposes, such a groundwater deficit would be an indicator of overexploitation of the aquifer on the assumption that the limits of the Riardo Plain hydrogeological system are a lateral, no flux boundary (corresponding to groundwater divides identified on the potentiometric map). This overall scenario should be reflected in the unavoidable aquifer depletion, as was observed in the last decades in other volcanic aquifers of central Italy (Capelli et al., 2005; Mazza and Mastrorillo, 2013; Mazza et al., 2014; Mazza et al., 2015). The case study of 23 years of monitoring of groundwater levels of the basal aquifer (Fig. 9) showed annual oscillations according to the seasonal variability of the recharge but not the expected decrease in the water table elevation. Indeed, Fig. 9 does not note any aquifer depletion, despite the outflows being much higher than the inflows. In addition, the near constancy of the Savone baseflow confirms the absence of a significant groundwater deficit. The minimum baseflow values measured at the beginning (July 1998: 315 L/s) and at the end (July 2013: 330 L/s) of the observation period are very similar. Furthermore, the qualitative comparison of the available minimum discharge rates of the Savone baseflow with the

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groundwater levels seems to depict a direct relationship (Fig. 9). Finally, although the total aquifer outflows (natural plus artificial) are greater than the calculated recharge, more than twenty years later, they still have not triggered depletion of the aquifer. Then, the presence of an additional lateral recharge from external hydrostructures can be supposed according to the hydrogeological setting of the study area. Nevertheless, the budget results allow the formulation of a first estimation of the external groundwater inflow that should be comparable at least to the budget discrepancy (i.e., approximately 485 L/s). This result means that the aquifer would present a combined recharge, deriving from the direct zenithal infiltration for approximately 60% and from external inflows for approximately 40%. The effects of the direct infiltration are evidenced by the relationship between the SPI12 calculated on the precipitation data of Riardo–Ferrarelle meteorological station and the groundwater level monitoring (Fig. 10). It is worth noting the response of the aquifer system to the precipitation regime. Starting from 1985, near normal or moderately dry periods were detected. An extremely dry period was detected from 1998 to 2002. The final period (2008/14) was characterized by a quick increase in the precipitation amount, defining extremely wet and very wet periods. The groundwater level trend shows annual oscillation according to the cyclic annual recharge and a progressive decrease until 2008, when the dry periods are followed by the first extremely wet period. Despite the significant changes in the precipitation, the groundwater level variations in the monitoring period were quite moderate. In fact, the volcanic portion of the basal aquifer, with a mean thickness of approximately 150 meters, presents a maximum groundwater level variation of approximately 4 meters, which corresponds to less than the 3% of the aquifer thickness. The same consideration could not be made for the carbonate sector of the basal aquifer because information regarding the total thickness of the limestone was not available. A thickness of approximately 80 meters of the saturated limestone was detected by borehole data (Giordano et al., 1995; Viaroli et al., 2016c). This very conservative information suggests a maximum variation of the aquifer thickness below 5%. These groundwater level oscillations result in lower than other overexploited porous or volcanic aquifers in southern Europe (Capelli et al, 2005; Custodio et al, 2016a; D’Alessandro et al., 2011; Kazakis, 2014, Mazza et al., 2014), in which tens of meters of drawdown were observed. This situation could suggest the presence of a continuous external contribution from neighboring hydrogeological systems that have good storage and self-regulation capabilities. These hydrodynamic attitudes are necessary to ensure a continuous and constant additional recharge capable of smoothing the large seasonal and annual changes in direct zenithal infiltration.

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The results obtained to date allowed the proposal of a new simplified conceptual model of the groundwater recharge of the Riardo Plain basal aquifer (Fig. 11). The structural setting of the sedimentary basement is characterized by a horst and graben structure due to the extensional tectonic activity, which dislocated the carbonate blocks. Most of the water infiltrated into the carbonate relief drains toward the main springs placed outside the study system. The remaining part of the water may supply the confined circulation in the deep fractured-carbonate blocks beneath the considered basal aquifer. According to the hydrogeological setting, the two aquifers can be clearly distinguished in the eastern sector of the plain, where the Flysch unit covers the carbonate substratum and plays the role of aquiclude. The study area is located in the wester sector of the plain, where the Flysch complex is not always present and a single groundwater circulation (“basal aquifer”) recharged by Roccamonfina volcano was identified (Viaroli et al., 2016c). This peculiar setting facilitates the inflow of underlying, semiconfined carbonate groundwater towards the overlying basal aquifer, where the difference in the hydraulic heads permits it. Therefore, it is reasonable that the studied aquifer could receive the greatest increase in recharge from the underlying carbonate units in correspondence with the mineral spring area, where the extensional fault systems and the absence of the intermediate aquiclude allows the uprising of deep fluids in correspondence with the concession area of the mineral water bottle plant (Cuoco et al., 2010). At present, it seems premature to assign an exact provenance to this external recharge. Nevertheless, it is reasonable to invoke the role of the carbonate ridges surrounding the Roccamonfina Volcano and the Riardo Plain, which host several productive aquifers, feeding spring systems outside the study area (Allocca et al., 2007; Boni et al, 1986). According to the literature, data about the effective infiltration values of these carbonate hydrostructures (ranging from 706 to 888 mm/year) indicate that the external recharge area needed to feed 485 L/s (seeming groundwater deficit) should cover between 16 to 20 km2. Such an extension is a small percentage of the total area of the carbonate hydrostructures surrounding the Riardo 2

Plain (more than 1000 km ); therefore, the identification of the recharge area based only on the groundwater budget is unlikely. The external water inflow amount is actually comparable with the uncertainties in the calculation of the carbonate hydrostructure budgets, which involve several m3/s. To prove the hypothesis suggested and to make the conceptual model more realistic, further investigation of both the local hydrogeological setting and the groundwater flow path of carbonate hydrostructures surrounding the study area is needed. For this purpose, a close cooperation between structural geologists and hydrogeochemists is desirable.

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6. Conclusions The results of the groundwater budget calculation can provide information about the sustainability of the anthropogenic activities on the groundwater resources. Combining the groundwater budget calculation with long-term aquifer monitoring, it was possible to provide preliminary indications about advice on the aquifer boundary conditions and information about the impact of the estimated withdrawals on the available groundwater resources. In fact, the groundwater budget calculation on the Riardo Plain aquifer highlights a significant groundwater deficit (approximately 40%), with no correspondence to any significant depletion of the groundwater resource as shown in the long-term aquifer monitoring. The length of the calculation period, more than twenty years (1992 – 2014), reasonably allows the exclusion of the effect of storage and transience of the study aquifer. Slight uncertainties regarding the appropriate identification of the recharge area are limited by the groundwater divides identified on the potentiometric map and the estimation of some budget parameters. These drawbacks do not justify such a great negative budget discrepancy. This anomaly could be explained by the presence of a hydraulic connection with a continuous external inflow from neighboring hydrostructures. The aquifers that provide the additional recharge should have a large extension, good storage and selfregulation capabilities in order to be able to smooth the effects of the intense variability of the local zenithal infiltration. The proposed hydrogeological model suggests an additional groundwater recharge, similar to the calculated groundwater deficit. This additional recharge comes from the underlying carbonate units, which in turn were recharged by the carbonate reliefs surrounding the Roccamonfina Volcano and the Riardo Plain. In correspondence with the mineralized spring area, extensional fault systems and the absence of the intermediate aquiclude make possible the rise and the mixing of groundwater under pressure. In conclusion, a groundwater budget deficit is not necessarily an indication of aquifer depletion but is evidence of external groundwater inflow, which has not been previously discussed. These first achievements provided the basis for the following research steps, the main objective of which would certainly be an identification of the exact provenance of the external recharge, generally ascribed to the surrounding carbonate reliefs. The complete understanding of the deep groundwater inflow requires other fundamental information 2

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inferable from stable isotope analysis (δ H, δ O), more detailed groundwater budget calculation and

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structural measurements of the surrounding carbonate reliefs. According to several authors (Baiocchi et al., 2013; La Vigna et al., 2016; Pola et al., 2015) numerical modelling could be a powerful tool to test conceptual models. In particular, in the study area, numerical models could help to define the dynamic relationship between the two overlapping aquifers and provide a more accurate quantification of the deeper inflow.

Acknowledgments The authors thank Dr. Emilio Cuoco for the continuous support and the precious debate on the Roccamonfina aquifers dynamics. The authors also thank Prof. Walter Dragoni for the precious suggestion on the use of the precipitation indexes. The authors also thank the Editor Prof Corradini, the Associate Editor and the two anonymous reviewers for their fundamental suggestion to improve the quality of the manuscript. This work was supported by Ferrarelle S.p.a. funding to the research project of the Roma Tre University on the Roccamonfina Volcano aquifer – Research supervisor: Prof. Roberto Mazza.

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Viaroli, S., Mastrorillo, L., Mazza, R., 2016b. Contribution of the Roccamonfina Caldera to the basal volcanic aquifer recharge: first considerations. Rend. Online Soc. Geol. It. 41, 95-98. doi:10.3301/ROL.2016.102 Viaroli, S., Mastrorillo, L., Mazza, R., Paolucci, V., 2016c. Hydrostructural setting of Riardo Plain: effects on Ferrarelle mineral water type. Italian Journal of Groundwater – Acque Sotterranee. 5 (3), 59-68. doi:10.7343/as-2016-226 Vicente-Serrano, S.M., López-Moreno, J.I., Beguería, S., Lorenzo-Lacruz, J., Azorín-Molina, C., MoránTejeda, E., 2012. Accurate computation of a streamflow drought Index. J. Hydrol. Eng. 17, 318–332. doi:10.1061/(ASCE)HE.1943-5584.0000433 World Meteorological Organization, 2012. Standardized Precipitation Index User Guide (M. Sodova, M. Hayes and D. Wood). WMO – No 1090, Geneva, 18 pp Zhou, Y., 2009. A critical review of groundwater budget myth, safe yield and sustainability. Journal of Hydrology. 370, 207-213. doi:10.1016/j.jhydrol.2009.03.009

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Figure captions Fig. 1 Map of the hydrostructures (modified from Boni et al., 1986). Legend: 1) Main springs (Q > 1 m3/s); 2) Streambed springs; 3) Groundwater flow direction; 4) Main rivers; 5) Main cities. Hydrostructures: G2) M. Simbruini, M. Ernici, M. Cairo, M. Camino, M. delle Mainarde and M. Cesima; S6) M Matese and M Totila; S7) Roccamonfina Volcano; U1) M. Maio; U2) M. Massico; U3) M. Maggiore. Fig. 2 Schematic hydrogeological cross section of the study area (modified from Capelli et al., 1999). Legend: 1) Basal aquifer water level; 2) Faults; 3) Pyroclastic deposits; 4) Mainly lava and minor pyroclastic deposits; 5) Flysch deposits; 6) Limestone units; 7) Springs; 8) Main groundwater flow directions. Fig. 3 Hydrogeological map of the study area. Legend: 1) Alluvial complex; 2) Coarse grained pyroclastic complex; 3) Fine grained pyroclastic complex; 4) Lava complex; 5) Flysch complex; 6) Limestone complex; 7) Dolomite complex; 8) Groundwater elevation map; 9) Streambed spring; 10) Hydrographic pattern; 11) Mainly irrigated area; 12) Recharge area; 13) Savone River closing station; 14) Main towns; 15) Teano – S. Giulianeta well field; 16) Mineral water bottling plant; 17)Weather stations: R – Roccamonfina station, F – Riardo Ferrarelle station. Fig. 4 Mean monthly thermo-pluviometric data in the 2000/14 period. Legend: Grey bars: mean monthly rainfall, Blue bars: mean monthly water surplus, Black dots: mean monthly temperature Fig. 5 A) C.I.P. values assigned to the hydrogeological complexes; B) Slope of the ground surface; C) C.I.P.S. values calculated for the cells of the study area Fig. 6 Distribution of the mean monthly effective infiltration Fig. 7 Monthly water surplus and water deficit in the 1992-14 period Fig. 8 SPI12 in the 1980/2014 period at the Riardo – Ferrarelle rain gauge. The dry and wet periods were defined according to the classification system realized by McKee et al. (1993). Legend 1) Extremely wet period; 2) Very wet period; 3) Near normal period; 4) Moderately dry period; 5) Extremely dry period Fig. 9 Long time groundwater level monitoring. PzC: carbonate aquifer portion; PzV: volcanic aquifer portion; Blue bars: Savone linear spring discharge Fig. 10 Relation between the PzC groundwater levels and the SPI12 computed in the Riardo – Ferrarelle rain gauge. The dry and wet periods were defined according to the classification system realized by McKee et al. (1993). Legend 1) Extremely wet period; 2) Very wet period; 3) Near normal period; 4) Moderately dry period; 5) Extremely dry period; 6) PzC groundwater level Fig. 11 Simplified conceptual scheme model of the Riardo Plain hydrogeological system. Not in scale. This is not a hydrogeological cross section. Legend: 1) Main carbonate springs; 2) Main discharge direction of the carbonate aquifers outside the plain; 3) Minor discharge directions in the phreatic carbonate aquifer; 4) Minor discharge directions in the confined carbonate aquifer; 5) Carbonate aquifer water level; 6) Volcanic aquifer water level; 7) Volcanic units; 8) Clays and flysch units; 9) Carbonate units.

Table captions Table 1 C.I.P. values assigned to the hydrogeological complexes Table 2 Mean monthly water surplus and deficit values calculated in the study area Table 3 Mean monthly effective infiltration values in the study area Table 4 Annual withdrawals and external water contribution provided by the Sannio – Alifano Consortium for agricultural purpose calculated from the monthly water deficit in the 1992/14 period. Table 5 Withdrawals for drinking use Table 6 Parameters involved in the groundwater budget calculation

30

31

32

33

34

35

36

37

38

39

40

41

Hydrogeological complex

C.I.P. (%)

Alluvial complex

90

Fine grained pyroclastic complex

45

Coarse grained pyroclastic complex

50

Lava complex

85

Limestone complex

95

January

February

March

April

May

June

July

August

September

October

November

December

TOTAL

Water surplus (mm)

94

71

68

48

0

0

0

0

0

0

75

111

467

Water deficit (mm)

0

0

0

0

0

0

-97

-102

0

0

0

0

-199

January

February

March

Water surplus (mm)

94

71

68

48

0

0

Effective infiltration (mm)

46

36

34

25

0

0

0

0

0

0

0

0

Water deficit (mm)

April

May

June

July

August

September

October

November

December

0

0

0

75

111

0

0

0

0

36

56

-97

-102

0

0

0

0

0

Year

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

20

Water deficit over the irrigate areas (L/s)

348

464

226

219

367

481

356

495

557

529

290

655

384

448

355

490

460

275

39

348

464

226

219

367

481

356

495

557

529

290

655

384

448

355

490

460

275

39

Sannio Alifano Consortium inflow (L/s) Agricultural withdrawals

42

(L/s)

Withdrawals for drinking use Town

Withdrawal (L/s)

Caianello

5

Marzano Appio

1

Riardo

7

Roccamonfina

10

Teano

36

Teano - Santa Giulianeta

240

Rocchetta e Croce

1

Total withdrawals

300

Recharge (L/s) Effective infiltration

740

Total inflows

740

Withdrawals (L/s) Drinking use

300

Industrial use

60

Agricultural use

395

Natural discharge (L/s) Savone Linear Spring Total outflows

470 1225

43

Highlights •

Groundwater budget calculation of an aquifer exploited for conjunctive purposes

•

Identification of a water deficit (40% of total outflows)

•

Comparison between the deficit and the long-term monitoring of the aquifer

•

Evaluation of the external deep groundwater inflow

•

Proposal of a conceptual model of the aquifer recharge

44